This invention relates to selected copolyimide compositions each of which can be processed as a melt and which exhibit recoverable crystallinity upon cooling from the melt. In preferred embodiments, these copolyimide compositions can also be produced in a melt via melt polymerization.
Polyimides constitute a class of valuable polymers being characterized by thermal stability, inert character, usual insolubility in even strong solvents, and high glass transition temperature (Tg) among others. Prior art discloses that their precursors have heretofore been polyamic acids, which may take the final imidized form either by thermal or chemical treatment.
Polyimides have always found a large number of applications requiring the aforementioned characteristics in numerous industries, and currently their applications continue to increase dramatically in electronic devices, especially as dielectrics.
Different aspects regarding polyimides and copolyimides may be found in a number of publications, such as for example:
Sroog, C. E., J. Polymer Sci.: Part C, No. 16 1191(1967).
Sroog, C. E., J. Polymer Sci.: Macromolecular Reviews, Vol. 11, 161 (1976).
Polyimides, edited by D. Wilson, H. D. Stenzenberger, and P. M. Hergenrother, Blackie, USA: Chapman and Hall, New York, 1990.
Several terms are defined below which are used in accordance with the present invention of high performance polyimides that possess simultaneously the following desirable properties: high thermal stability, such that they can be processed in the melt, and which exhibit recoverable semicrystallinity upon crystallization from the melt.
The term xe2x80x9cmelt-processible polyimidexe2x80x9d means that the polyimide has sufficiently high thermoxidative stability and sufficiently low melt viscosity at temperatures at or above the melting point of the polyimide such that the polyimide can be processed in the melt to form a shaped object (e.g., extruded into a pellet, etc.) without the polyimide undergoing any significant degradation.
The term xe2x80x9cmelt-polymerizable polyimidexe2x80x9d means that the polyimide can be formed in a melt in the absence of solvent by reaction of its respective monomers (e.g., dianhydride(s) and diamine(s)) to form initially polyamic acid(s), which are subsequently converted to the polyimide. Furthermore, the polyimide produced has sufficiently high thermoxidative stability and sufficiently low melt viscosity at temperatures at or above the melting point of the polyimide such that the polyimide can be processed in the melt to form a shaped object (e.g., extruded into a pellet, etc.) without the polyimide undergoing any significant degradation.
The term xe2x80x9cDSCxe2x80x9d is an acronym for differential scanning calorimetry, a thermal analysis technique widely used for accurately determining various thermal characteristics of samples, including melting point, crystallization point, and glass transition temperature. The acronym xe2x80x9cDSCxe2x80x9d is employed in text that follows below. The following definitions of slow, intermediate, and fast crystallization kinetics and related terms are based upon behavior of a given sample during DSC analysis under slow cooling, quench cooling, reheat, etc. scans during the DSC analysis (see infra for details).
The term xe2x80x9cslow crystallization kineticsxe2x80x9d means that the crystallization kinetics are such that, for a given copolyimide sample, the sample, when subjected to DSC analysis, essentially does not show any crystallization during slow cooling (i.e., cooling at 10xc2x0 C./minute) from its melt but does exhibit a crystallization peak on subsequent reheat. Furthermore, no crystallization occurs upon quench cooling.
The term xe2x80x9cintermediate crystallization kineticsxe2x80x9d means that the crystallization kinetics are such that, for a given copolyimide sample, when subjected to DSC analysis, the sample exhibits some crystallization on slow cooling and furthermore does exhibit some crystallization on reheat after slow cooling. Furthermore, there is no strong evidence for crystallization occurring upon quench cooling.
The term xe2x80x9cfast crystallization kineticsxe2x80x9d means that the crystallization kinetics are such that, for a given copolyimide sample, when subjected to DSC analysis the sample does exhibit crystallization peaks in both slow and quench cooling and furthermore no observable crystallization peak is seen on subsequent reheat of a given sample following slow cooling. After quench cooling, there may be some crystallization exhibited on reheat.
The term xe2x80x9cmelt of a polymerxe2x80x9d means the polymer exists as the melt in a liquid or substantially liquid state. If the polymer is crystalline or semicrystalline, a melt of the polymer is necessarily at a temperature greater than or equal to its melting point (Tm).
The term xe2x80x9crecoverable semicrystallinityxe2x80x9d and/or xe2x80x9crecoverable crystallinityxe2x80x9d refers to behavior occurring in a semicrystalline or crystalline polymer and specifically means that behavior that occurs when the polymer, upon heating to a temperature above its melting point and subsequent slow cooling to a temperature well below its melting point, exhibits a melting point in a reheat DSC scan. (If a melting point is not observed during the reheat DSC scan, the polymer does not exhibit recoverable crystallinity. The longer a sample is below Tm but above Tg, the greater probability it has to crystallize.)
The term xe2x80x9csemicrystalline polymerxe2x80x9d means a polymer that exhibits at least some crystalline characteristics and is partially but not completely crystalline. Most or all known polymers having crystalline characteristics are semicrystalline, but not totally crystalline, since they also have at least some amorphous characteristics. (Hence the term crystalline polymer is technically a misnomer in most or all instances where it is used, but nevertheless is often used.)
The melt index of a polymer is defined to be the number of grams of polymer extruded at a specific temperature and load through a die of a specified length and diameter in a time period of ten minutes. Details of the geometry and test procedures are described in ASTM D1238. (ASTM=American Society for Testing and Materials).
Some significant advantages of melt processing of semicrystalline polyimides having recoverable crystallinity according to the invention include processing without a solvent such that tedious and costly solvent recycling is unnecessary and can be eliminated. High thermal stability is not only essential for processing in the melt at temperatures of greater than or equal to 350xc2x0 C. but also is required for polyimides used in high temperature applications. Semicrystalline polyimides are often highly desirable in comparison to otherwise comparable polyimides that are amorphous, since the former in relation to the latter often exhibit superior properties, such as having better mechanical properties (e.g., especially higher modulus), capability for use at higher temperatures without property degradation (e.g., better solder resistance, modulus retention), higher solvent resistance, higher creep viscosities (e.g., lower tendencies for distortion of a film or other structure with time), and lower coefficients of thermal expansion.
In order for a semicrystalline polyimide to be considered melt-processible, the polyimide must possess a melting point below a temperature of about 385xc2x0 C., which temperature is a practical limit for melt processing due to both equipment capabilities/limitations and to avoid any significant thermal degradation of the polyimide. Furthermore, the polyimide also must possess a sufficiently low melt viscosity (i.e., less than or equal to a maximum of about 108 poise (which is equal to 107 Pascal-seconds), but preferably 104 poise (which is equal to 103 Pascal-seconds), depending on polymer melt temperature and shear rates of the melt processing equipment). Copolymerization can be used to lower the melting temperature of a polymer (e.g., polyimide) but usually results in loss of crystallinity. Prior art compositions have been unable to achieve suitable reduction in the melting points (Tms) of the copolymeric compositions while simultaneously maintaining substantial degrees of semi-crystallinity in the copolymeric compositions. In the compositions of this invention, both suitable melting temperatures and high degrees of semi-crystallinity are achieved by judicious choice of comonomers and their relative amounts in the compositions.
Polyimides that exhibit a melting point in an initial DSC heat scan and which are thereby attributed to have crystalline characteristics are disclosed in Kunimune, U.S. Pat. No. 4,923,968 to Chisso Corporation. While the copolyimides disclosed in this patent may be crystalline or semicrystalline until heated to temperatures above their melting points, the present inventors have not observed the copolyimides disclosed in this patent likely to exhibit recoverable crystallinity. Indeed these copolyimides are probably substantially amorphous when cooled from their melts. Furthermore, many of the copolyimides disclosed in this patent are not melt-processible, because they have melting points, molecular weights, and/or melt viscosities that are too high for melt-processibility. In addition, endcapping in order to moderate the polymerization and improve melt processibility is not taught.
The selected random copolyimides of this invention overcome the drawbacks of the prior art compositions in that these copolyimides possess simultaneously these key essential propertiesxe2x80x94high thermal stability, melt-processibility, and recoverable crystallinity. The copolyimides of this invention can therefore be processed in the melt to form articles, which may have a predetermined shape, such as extrudates, fibers, films, and molded products comprised of these semicrystalline copolyimides. In many cases, the copolyimides of this invention can also be produced in the melt (via melt-polymerization).
There is a significant long-felt need not met by the current state of polyimide art for high performance polyimides that possess high thermal stability, which can be processed in the melt (melt-processible), and which exhibit recoverable semicrystallinity upon crystallization from the melt. This invention provides a solution to this long-felt need. There is also a long-felt need not met by the current state of polyimide art for high performance polyimides that can be produced by melt polymerization of appropriate monomers in a melt. In many embodiments, this invention also provides a solution to this latter long-felt need.
In one embodiment, the invention is a melt-processible, thermoplastic copolyimide comprising the reaction product of components comprising:
(I) an aromatic dianhydride component consisting essentially of 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) in combination;
(II) an aromatic diamine component selected from the group consisting of 1,3-bis(4-aminophenoxy) benzene (APB-134); 3,4xe2x80x2-oxydianiline (3,4xe2x80x2-ODA); 3,4xe2x80x2-oxydianiline (3,4xe2x80x2-ODA) and 4,4xe2x80x2-oxydianiline (4,4xe2x80x2-ODA) in combination; 3,4xe2x80x2-oxydianiline (3,4xe2x80x2-ODA) and 1,4-diaminobenzene (PPD) in combination; 1,3-bis(4-aminophenoxy) benzene (APB-134) and 4,4xe2x80x2-oxydianiline (4,4xe2x80x2-ODA) in combination; and 1,3-bis(4-aminophenoxy) benzene (APB-134) and 1,4-diaminobenzene (PPD) in combination; and
(III) an endcapping component; wherein the copolyimide has a stoichiometry in the range from 93% to 98%, has a molar ratio of 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride/pyromellitic dianhydride (BPDA/PMDA) of greater than or equal to 70/30 but less than or equal to 95/5, exhibits a melting point in the range of 330xc2x0 C. to 385xc2x0 C., and exhibits recoverable crystallinity as determined by differential scanning calorimetry analysis.
As used herein the term xe2x80x9cstoichiometryxe2x80x9d, expressed as a percent, means total moles of dianhydride(s) in relation to total moles of diamine(s) that are incorporated in a given polyimide. If the total moles of dianhydride(s) equals the total moles of diamine(s), the stoichiometry is 100 percent. If these two numbers are not equal, either total diamine(s) or total dianhydride(s) is present in higher amount, and the stoichiometery in this case is expressed as the mole percentage of component(s) (diamine(s) or dianhydride(s)) present in lesser amount relative to that component(s) present in higher amount. As one example, if a polyimide sample is derived from incorporation of 0.98 mole of dianhydride(s) and 1.00 mole of diamine(s), the diamine(s) is present in higher amount and the stoichiometery is 98%.
As used herein the term xe2x80x9cendcappingxe2x80x9d refers to the monofunctional component(s) (agent(s)) including, but not limited to, phthalic anhydride, naphthalic anhydride, and aniline, which cap the copolyimides to moderate the polymerization and to enhance thermoplasticity of the final melt polymerized product. Endcapping is generally done to 100% such that total moles of anhydride functionality are equal to total moles of amine functionality. Phthalic anhydride and naphthalic anhydride are suitable endcapping components in those cases where diamines are present in greater molar amounts than are dianhydrides. Aniline is a suitable endcapping component in those cases where dianhydrides are present in greater molar amounts than are diamines. The percentage of endcapping component required to afford 100% endcapping is equal to twice the value of (1-stoichiometry) multipled by 100. As an example, for a 100% endcapped copolyimide with 95% stoichiometry (diamine in excess), the total moles of the endcapping agent must be 10 mole percent of the total moles of the diamines, i.e., 10 moles of the endcapping agent to 100 moles of the diamines.
A given melt-processible copolyimide of the invention can in most instances be obtained by melt-polymerization or, alternatively, in all instances by traditional solution polymerization techniques, the latter of which are well known in the art. The melt processing technique of the invention can be used to manufacture an article of predetermined shape.
In the melt polymerization technique, the method of the invention comprises the steps of:
(a) blending, to substantial homogeneity, components comprising:
(I) 93 to 98 mole parts of an aromatic dianhydride component consisting essentially of 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) in combination;
(II) 100 mole parts of an aromatic diamine component selected from the group consisting of 1,3-bis(4-aminophenoxy)benzene (APB-134),3,4xe2x80x2-oxydianiline (3,4xe2x80x2-ODA); 3,4xe2x80x2-oxydianiline (3,4xe2x80x2-ODA) and 4,4xe2x80x2-oxydianiline (4,4xe2x80x2-ODA) in combination; 3,4xe2x80x2-oxydianiline (3,4xe2x80x2-ODA) and 1,4-diaminobenzene (PPD) in combination; 1,3-bis(4-aminophenoxy)benzene (APB-134) and 4,4xe2x80x2-oxydianiline (4,4xe2x80x2-ODA) in combination; and 1,3-bis(4-aminophenoxy)benzene (APB-134) and 1,4-diaminobenzene (PPD) in combination; and
(III) 4 to 14 mole parts of at least one endcapping component;
the components (I), (II) and (III) being in substantially solventless form and the blending step producing a substantially solventless component blend;
the blending step being carried out at a temperature below the melting point of any of components (I), (II) and (III);
the component (I) and (II) being present in the component blend in a molar ratio of (I):(II) from 0.93 to 0.98;
the component (III) being present in the component blend in a molar ratio (III):(II) of 0.04 to 0.14;
(b) heating the substantially solventless component blend produced in step (a) to a predetermined melt processing temperature at which the (I) aromatic dianhydride component and the (II) aromatic diamine component are melted and will react to form a melt of a polyimide; the predetermined melt processing temperature being less than the temperature at which the polyimide melt chemically decomposes;
(c) mixing the component blend and the polyimide melt produced therefrom during the heating step (b);
(d) removing water of reaction from the component blend and the polyimide melt produced therefrom during the heating step (b);
(e) forming the polyimide melt into an article having predetermined shape; and
(f) cooling the article having predetermined shape to ambient temperature;
wherein the aromatic dianhydride component has a molar ratio of 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride/pyromellitic dianhydride (BPDA/PMDA) of greater than or equal to 70/30 but less than or equal to 95/5, the polyimide exhibits a melting point in the range of 330xc2x0 C. to 385xc2x0 C., and the polyimide exhibits recoverable crystallinity as determined by DSC analysis.